kconfig: use 'else ifneq' for Makefile to improve readability

'ifeq ... else ifneq ... endif' notation is supported by GNU Make 3.81
or later, which is the requirement for building the kernel since
commit 37d69ee3 ("docs: bump minimal GNU Make version to 3.81").

Use it to improve the readability.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/kconfig/Makefile

@@ -76,15 +76,13 @@ savedefconfig: $(obj)/conf
 defconfig: $(obj)/conf
 ifeq ($(KBUILD_DEFCONFIG),)
 	$< $(silent) --defconfig $(Kconfig)
-else
-ifneq ($(wildcard $(srctree)/arch/$(SRCARCH)/configs/$(KBUILD_DEFCONFIG)),)
+else ifneq ($(wildcard $(srctree)/arch/$(SRCARCH)/configs/$(KBUILD_DEFCONFIG)),)
 	@$(kecho) "*** Default configuration is based on '$(KBUILD_DEFCONFIG)'"
 	$(Q)$< $(silent) --defconfig=arch/$(SRCARCH)/configs/$(KBUILD_DEFCONFIG) $(Kconfig)
 else
 	@$(kecho) "*** Default configuration is based on target '$(KBUILD_DEFCONFIG)'"
 	$(Q)$(MAKE) -f $(srctree)/Makefile $(KBUILD_DEFCONFIG)
 endif
-endif
 
 %_defconfig: $(obj)/conf
 	$(Q)$< $(silent) --defconfig=arch/$(SRCARCH)/configs/$@ $(Kconfig)